The present invention relates to power tools, and more particularly to stop controllers which are used for stopping the operation of power tools, such as boring tools, when the tool comes into contact with metal embedded within a building structure. The metal could be structural metal, reinforcing rod, conduit, high voltage wires, or the like.
In the building and construction industries it is frequently necessary during construction and backfit operations to drill and/or cut into existing structures, whether buildings, building walls, building slabs, pavement, etc. Many, if not most, of these structures contain embedded metal. The metal may be reinforcing steel, structural steel, duct work, plaster lath, water pipes, drain pipes, gas pipes, electrical conduits, electrical wires themselves, and so forth. It is, of course, very important to stop the operation of such a tool the very moment it contacts such a metallic object, before the cutting bit of the tool can cause damage to either the metal object or to the tool bit itself. Not only should such building structural and service elements be protected from damage, but even more importantly, the risk to the tool operator of electrical shock from penetrated electrical lines or conduit, of gas explosions from penetrated gas lines, and so forth, must be minimized. A quick response is also of great importance to lessen the risk of worker accidents or injury from a tool which might otherwise slip or jump from the operator's control when a still moving tool bit suddenly becomes bound in embedded metal in the building structure.
In certain tool stop controller designs, a circuit interrupter or power circuit shutdown device includes circuitry which is actually interposed in the line between the power line ground and the tool ground. A typical example is shown in U.S. Pat. No. 3,386,004 (Dwyer, issued May 28, 1968). Circuitry such as described therein detects contact of the tool with embedded structural metal by placing a low impedance, low voltage source on the tool ground line and then detecting equipment ground lead undervoltage caused by subsequent tool contact with the embedded metal. Upon detecting such contact, the protection circuit is quickly actuated to stop operation of the tool by interrupting transmission of further power thereto. Frequently, the protection circuit also restores the ground circuit connection upon such a shutdown.
With many such tools, a proper connection to the original ground at the power source is necessary for protecting the tool operator by sinking leakage and fault currents. When used with many such tool stop controllers, such a proper connection to the power source ground is also necessary for providing the voltage reference for the associated tool stop controller shutdown circuitry. Even if the tool stop controller circuit is of a type wherein the ground circuit from the tool housing to the power line ground is not directly interrupted by the stop controller circuit, it is still obviously very important to maintain ground circuit continuity for sinking leakage and fault currents (and, depending upon the circuit design, ground circuit continuity may still be necessary for proper circuit operation).
As is well known, the transmitted power (e.g., 120 VAC) in a single phase, three conductor power line is applied across the hot and neutral lines (or leads). Both the neutral and ground leads are connected to earth ground at the power line source (and often at additional locations along the line as well). The ground lead is intended for safety purposes and normally does not carry any current.
To fulfill this safety purpose, the ground lead is ordinarily connected to the power tool housing, and preferably to other exposed metallic parts of the power equipment. Upon contact with energized building or structural metal, or upon internal failure within the tool itself, voltages impressed upon the tool housing will then ordinarily be conducted to ground by the ground lead, protecting the tool operator from dangerous exposure thereto. Obviously therefore, overall proper operation of the tool stop controller is dependent upon the integrity of the ground connection through the power line ground lead.
Ground fault detectors are well known, of course. Perhaps the simplest is a mere neon lamp and load resistor connected in series between the hot and ground leads. Should the ground lead fail, the lamp will fail to light. However, in such a simple circuit an illuminated lamp does not always mean that the ground lead connection is good. For example, the lamp will also usually light if the circuit is miswired, as by reversing the hot and ground leads. Therefore, more sophisticated detectors have been devised, such as shown and described in U.S. Pat. No. 4,298,864 (Mahnke et al., issued Nov. 3, 1981).
When viewed from the environment of tool stop controllers, however, many of the newer and more sensitive detectors have also proved to be unsatisfactory. That is, if another ground path is present, even if of high impedance, such as through the body of a worker contacting the tool housing, a "good" or "safe" indication will often be provided even though the actual ground lead is faulty (open circuited). In some cases also, an unsafe voltage may be imposed upon the tool housing by the detector circuitry once the ground lead becomes faulty, thereby exposing the tool operator to a potentially hazardous shock.
A need therefore remains for an improved ground continuity monitor particularly well adapted for use with tool stop controllers. Such a tool stop controller ground continuity monitor should be able, independently of the controller, to monitor the proper continuity of the equipment ground lead back to the power line source. Such a monitor should also be able to distinguish correctly between a proper low impedance ground path to the power line source and a false high impedance path through a worker's hand and body to ground if the equipment ground path is open. In the latter case, wherein the equipment ground path is open, the monitor circuit should impose no more than a relatively safe, low voltage, high impedance signal on the tool case and tool operator. Such a monitor should also provide an indication when the proper ground continuity is present. Further, it should preferably be able as well to monitor problems associated with miswiring from the power supply, such as reversed polarity, open neutral or hot, and reversed hot and ground, as well as the open ground condition itself.